Optical materials and optical products are useful to control the flow and intensity of light. Examples of useful optical products include optical lenses such as Fresnel lenses, optical light fibers, light tubes, optical films including totally internal reflecting films, retroreflective sheeting, and microreplicated products such as brightness enhancing films (BEF) and security products. Examples of some of these products are described in U.S. Pat. Nos. 4,542,449; 5,175,030; 5,591,527; 5,394,255 and others.
Optical products can be prepared from high index of refraction materials, including monomers such as high index of refraction (meth)acrylate monomers, halogenated monomers, and other such high index of refraction monomers as are known in the art. See, for example, U.S. Pat. Nos. 4,568,445; 4,721,377; 4,812,032; and 5,424,339.
The monomers can be cured or polymerized to take the form of a product capable of modifying or controlling the flow of light. In the particular structure of a microreplicated optical product, the monomers can be polymerized into a brightness enhancement film having a micro-fine prismatic pattern. See U.S. Pat. Nos. 5,175,030 and 5,183,597. Brightness enhancement films are very useful in many of today's electronic products to increase the brightness of backlit flat panel displays such as liquid crystal displays (LCDs), electroluminescent panels, laptop computer displays, word processors, desktop monitors, televisions, video cameras, and automotive and avionic displays, among others.
One important property of an optical material is its index of refraction, because index of refraction is related to how effectively an optical material can control the flow of light. There exists a continuing need for optical materials and optical products that exhibit a high index of refraction.
With respect specifically to brightness enhancement films, the index of refraction is related to the brightness gain or “gain” produced by the brightness enhancement film. Gain is a measure of the improvement in brightness of a display due to the brightness enhancement film, and is a property of the optical material, and also of the geometry of the brightness enhancement film. Typically, the viewing angle decreases as the gain increases. A high gain is desired for a brightness enhancement film because improved gain provides an effective increase in the brightness of a backlit display.
Improved brightness means that the electronic product can operate more efficiently by using less power to light the display. Reduced power consumption translates into reduced heat generation and therefore means increased component life. Thus, because of these advantages, there exists a continuing need to find optical products exhibiting improved index of refraction values.
Maruyama et al., U.S. Pat. No. 5,183,917, describes the synthesis of a diphenyl sulfide of the formula: This compound is described as having a high index of refraction. However, this monomer (4, 4′-bis(methacroyl thio)diphenyl sulfide, or MPSMA) is known to be a crystalline solid that is not very soluble in common acrylate monomers. Since a liquid composition is necessary to make a microreplicated product, MPSMA has had limited usefulness.
Fukushima et al., U.S. Pat. No. 5,969,867, recognized the usefulness of MPSMA, but found it necessary to use bis(methacryloxyethoxyphenyl) propane and similar materials to solubilize the MPSMA. Unfortunately, these solubilizing materials have relatively low index of refraction values and thus it is difficult to produce compositions having index of refraction values above about 1.60 without using very high levels of MPSMA.
However, it is preferred to use lower levels of MPSMA due to its high cost and limited compatibility. In a particular example, Fukushima was able to obtain a composition with a refractive index of 1.65 that contained only 50 percent MPSMA by using a high level of phenyl thioethoxyethyl methacrylate. This material is not believed to be available commercially and is difficult to synthesize.
Thus, a need remains for improved polymerizable compositions having high index of refraction values using materials that can be easily synthesized using commercially available starting materials.